Can galactic magnetic fields diffuse into the voids?

TL;DR

The paper argues that cosmic voids are not vacuum but conducting plasmas, which makes diffusion of magnetic fields from galaxies extremely inefficient. Through analytical considerations and mean-field MHD simulations, it shows that even with turbulent diffusion, the outward spread of dynamo-generated galactic fields into voids remains limited to sub-Mpc scales within a Hubble time, and quadrupolar configurations decay as $\sim r^{-2}$ inside the diffusion radius, forming a magnetized magnetosphere around galaxies. Consequently, filling the vast volumes of voids with magnetization by astrophysical processes alone is implausible, favoring primordial magnetogenesis as the natural explanation for space-filling weak void fields. The work also outlines observational signatures via Faraday rotation measures, indicating current data are close to, but generally below, detection thresholds, while future probes (e.g., FRB RM studies) could help discriminate between primordial and astrophysical origins.

Abstract

Cosmic voids are magnetized at the level of at least $10^{-17}$ G on Mpc scales, as implied by blazar observations. We show that an electrically conducting plasma is present in the voids, and that, because of the plasma, \emph{diffusion} into the voids of galactic fields generated by a mean-field dynamo is far too slow to explain the present-day void magnetization. Indeed, we show that even in the presence of turbulence in the voids, dynamo-generated galactic fields diffuse out to a galactocentric radius of only 200-400 kpc. Therefore, it is challenging to meet the required volume filling-factor of the void magnetic field. We conclude that a primordial origin remains the most natural explanation to the space-filling weak fields in voids.

Figures (6)

• Figure 1: Upper plot: conductivity of the Universe (in natural units) as a function of redshift (blue solid line) together with Spitzer conductivity Eq. (\ref{['eq:spitzer']}), i.e. Coulomb scattering of the electrons (orange dashed line). Lower plot: resistivity $\eta=(\mu_0\sigma)^{-1}$ as a function of redshift (blue solid line), together with the ratio between the diffusion time and the Hubble times, evaluates at the Hubble scale (orange dashed line; see Sec. \ref{['SpreadingByDiffusion']}).
• Figure 2: Logarithm of the quadrupolar $|{\overline{\bm{B}}}|$ versus time and radius showing the diffusive expansion for $\eta_{\rm turb}^{\rm ext}=10^{30}\,{\rm cm}^2\,{\rm s}^{-1}$ (top) and $2\times10^{29}\,{\rm cm}^2\,{\rm s}^{-1}$ (bottom). The lower black dashed and upper white dashed lines denote $\ell_\text{diff}=[q_{\rm diff}\eta_{\rm turb}^{\rm ext}(t-t_\ast)]^{1/2}$ with $q_{\rm diff}=2$ and 100, respectively.
• Figure 3: Radial profiles of RM for the quadrupolar field: we observe a power-law behavior of RM for $r<r_*\simeq 300$ kpc, and a faster decay otherwise. The dashed-dotted line indicates the $r^{-2}$ scaling.
• Figure 4: Plot of RM for a dipolar (upper panel) and quadrupolar (lower panel) field. The dotted lines denote radial cuts through $\theta=30^\circ$, $45^\circ$, and $60^\circ$: the RM along these lines is shown in Fig. \ref{['prm_Qdiag']} for the quadrupolar field.
• Figure 5: The universe is a collisional plasma at cosmological scales throughout its thermal history. Upper panel: plasma parameter as a function of redshift. Middle panel: ratio of the mean free path of protons (orange, solid curve) and electrons (blue, dashed curve) to the Debye length as a function of redshift. Bottom panel: ratio of the mean free path of protons (orange, solid curve) and electrons (blue, dashed curve) to the Hubble length as a function of redshift.